Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy from wind using known foil principles and transmit the kinetic energy through rotational energy to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
To ensure that wind power remains a viable energy source, efforts have been made to increase energy outputs by modifying the size and capacity of wind turbines. One such modification has been to increase the length and surface area of the rotor blades. However, the magnitude of deflection forces and loading of a rotor blade is generally a function of blade length, along with wind speed, turbine operating states, blade stiffness, and other variables. This increased loading not only produces fatigue on the rotor blades and other wind turbine components but may also increase the risk of a sudden catastrophic failure of the rotor blades, for example when excess loading causes deflection of a blade resulting in a tower strike.
Load control is thus a crucial consideration in operation of modern wind turbines. For example, active pitch control systems are widely used to reduce the load on the rotor blades by varying the pitch of the blades in high load conditions. U.S. Pat. Appln. Pub. No. 2010/0092288 describes a pitch-adjustable blade with a root-end bearing having a pitch axis that is angled relative to the longitudinal axis of the blade. With this configuration, the blade moves into and out of the rotor plane when pitched, which essentially changes the effective length of the blade. It is also known to vary the aerodynamic characteristics of the individual rotor blades as a means of load control, for example with controllable vortex elements, flaps, tabs, and the like configured on the blade surfaces.
U.S. Pat. No. 6,972,498 describes various wind turbine blade configurations wherein a retractable extension is provided on a base blade segment to reduce the effective length of the blade in high load conditions. In a particular embodiment, the blade extension is hinged to the base blade segment and jackknifes between a fully extended position and a fully retracted position wherein the blade extension folds into the base blade segment. The articulation joint for this type of arrangement results in open sections along the blade edge, which could produce noise and adversely affect the aerodynamic performance of the blade in the retracted (partially or fully) position of the blade extension.
Accordingly, the industry would benefit from an improved system for reducing the effective length of a wind turbine rotor blade in high load conditions.